Loss of endocytosis-associated RabGEF1 causes aberrant morphogenesis and altered autophagy in photoreceptors leading to retinal degeneration

Abstract

Rab-GTPases and associated effectors mediate cargo transport through the endomembrane system of eukaryotic cells, regulating key processes such as membrane turnover, signal transduction, protein recycling and degradation. Using developmental transcriptome data, we identified Rabgef1 (encoding the protein RabGEF1 or Rabex-5) as the only gene associated with Rab GTPases that exhibited strong concordance with retinal photoreceptor differentiation. Loss of Rabgef1 in mice (Rabgef1-/-) resulted in defects specifically of photoreceptor morphology and almost complete loss of both rod and cone function as early as eye opening; however, aberrant outer segment formation could only partly account for visual function deficits. RabGEF1 protein in retinal photoreceptors interacts with Rabaptin-5, and RabGEF1 absence leads to reduction of early endosomes consistent with studies in other mammalian cells and tissues. Electron microscopy analyses reveal abnormal accumulation of macromolecular aggregates in autophagosome-like vacuoles and enhanced immunostaining for LC3A/B and p62 in Rabgef1-/- photoreceptors, consistent with compromised autophagy. Transcriptome analysis of the developing Rabgef1-/- retina reveals altered expression of 2469 genes related to multiple pathways including phototransduction, mitochondria, oxidative stress and endocytosis, suggesting an early trajectory of photoreceptor cell death. Our results implicate an essential role of the RabGEF1-modulated endocytic and autophagic pathways in photoreceptor differentiation and homeostasis. We propose that RabGEF1 and associated components are potential candidates for syndromic traits that include a retinopathy phenotype.

Reserpine maintains photoreceptor survival in retinal ciliopathy by resolving proteostasis imbalance and ciliogenesis defects

“…Autophagy is a fundamental homeostatic pathway that serves as a key quality control mechanism for degradation and recycling of components in all tissues, including highly metabolic tissues such as photoreceptors (99, 100). Reserpine's reported role as an autophagy modulator (60, 101) is consistent with photoreceptor survival in general (99,(102)(103)(104)(105), and especially in CEP290 disease because of the purported connection between autophagy and ciliogenesis (71,72). Ciliary defects in photoreceptors promote apoptotic cell death through accumulation of outer segment proteins (e.g., opsin) in the endoplasmic reticulum (ER) and subsequent activation of the unfolded protein response (UPR) (1) to restore protein homeostasis in ER.…”

“…We note that inhibition of autophagy or activation of proteasome activity can maintain photoreceptors survival in mouse models with misfolded rhodopsin ( 104 ). As autophagy defects are associated with retinal degeneration ( 99, 105 ), reserpine holds promise to serve as a gene agnostic therapy. Interestingly, a favorable effect of autophagy inhibition has also been reported for retinal ganglion cell survival ( 107 ).…”

Pathways controlling neurotoxicity and proteostasis in mitochondrial complex I deficiency

“…Aberrant proteostasis has been strongly implicated in the age-related disorders macular degeneration, cataract and glaucoma [ 54 ]. Alterations in the ubiquitin/proteasome and autophagy/lysosome systems have been documented in various genetic forms of retinal degeneration and in other aging-dependent ocular diseases [ 54 , 55 ], as has oxidative stress and mitochondrial dysfunction [ 42 , 43 ], suggesting fundamental connections among these pathways. A functionally important relationship between altered proteostasis and photoreceptor vulnerability was recently suggested by a chemical screening approach in an induced pluripotent stem cell-derived model of retinal degeneration caused by genetic ciliary dysfunction, which identified reserpine as both promoting photoreceptor survival and normalizing proteostasis [ 56 ].…”

Pathways controlling neurotoxicity and proteostasis in mitochondrial complex I deficiency

“…Aberrant proteostasis has been strongly implicated in the age-related disorders macular degeneration, cataract and glaucoma (54). Alterations in the ubiquitin/proteasome and autophagy/lysosome systems have been documented in various genetic forms of retinal degeneration and in other aging-dependent ocular diseases (54,55), as has oxidative stress and mitochondrial dysfunction (42,43), suggesting fundamental connections among these pathways. A functionally important relationship between altered proteostasis and photoreceptor vulnerability was recently suggested by a chemical screening approach in an induced pluripotent stem cell-derived model of retinal degeneration caused by genetic ciliary dysfunction, which identified reserpine as both promoting photoreceptor survival and normalizing proteostasis (56).…”